Many grapple type log skidders used in forestry operations have a grapple supported from a boom mounted to the rear of the skidder. The grapple commonly includes a pair of opposed tongs which may be actuated by a hydraulic actuator to open and close around a group of logs. The boom is also commonly controlled by another hydraulic actuator for lowering and raising the grapple. The more sophisticated loaders may even have a mechanism for rotating the grapple relative to the boom and/or tilting the grapple relative to the boom.
Typically in operation, the grapple boom is lowered with the grapple tongs open to engage a group of logs and hydraulic pressure is then applied to the actuator to close the grapple tongs about the group of logs. The boom is then raised to lift a forward end of the logs off the ground so that the logs can be dragged from the cut site to the processing site.
One of the problems in the above operation is that when the logs are being dragged by the skidder, the logs have a tendency to slip creating a smaller mass about which the grapple tongs are closed. Such shifting could cause the grapple tongs to lose their grip on the logs sufficient to permit the loss of one or more of the logs. Thus, the operator had to diligently watch the logs and manually shift the control valve to readjust the grapple tongs when the grip on the logs loosened.
One of the early attempts to solve this problem included adding a detent to hold the grapple control valve in the grapple closed position during the dragging operation. However, if the grapple control valve of an open center grapple circuit is held in the grapple closed position, the hydraulic system overheats due to the full pump flow passing through a relief valve. That also consumes large amounts of power thereby reducing the available power to the drivetrain.
The overheating problem is reduced somewhat through the use of closed centered a load sensing hydraulic system in which the displacement of a variable displacement load sensing pump is reduced to a high pressure standby position when the grapple control valve is maintained in the grapple closed position. However, since the size of the variable displacement pump is normally selected to provide adequate flow for fast response time when more than one actuator is actuated at the same time, a considerable amount of heat is still generated in the system when the pump is maintained at a high pressure standby position for an extended period of time such as during a dragging operation.
Thus, it would be desirable to provide a hydraulic system for operating a grapple mechanism of a skidder wherein the hydraulic system has sufficient fluid flow capability for providing fast response of the actuators while providing positive fluid pressure to the actuator controlling the grapple tongs without generating excessive heat in the hydraulic system. Maintaining a positive fluid pressure on the actuator would automatically cause the actuator to readjust the position of the grapple tongs when the closing force on the tongs loosened due to shifting of the logs within the grapple.
The present invention is directed to overcoming one or more of the problems as set forth above.